Recently, the packaging density of an electronic circuit has been greater, and a circuit board and a device have been reduced in size by the use of a ceramic multilayer circuit board and a flexible printed circuit (FPC). In a conventional technique for connecting an FPC to either a ceramic circuit board or a printed board, connectors are mounted previously on the boards so as to provide a connection between the connectors, or the FPC is inserted into the connectors. As for a circuit board in which a mounting area or height of the connectors cannot be ensured, solder is used for the connection.
Hereinafter, a conventional connection method using solder will be described with reference to FIG. 3. In FIG. 3, reference numerals 1, 2, 3, 4 and 5 denote a ceramic circuit board, a circuit conductor on the ceramic circuit board, solder for connection, a circuit conductor on an FPC, and an FPC, respectively.
Conventionally, the ceramic circuit board is connected to the FPC using solder as follows. Cream solder is printed previously on the connecting circuit conductor on the ceramic circuit board. Afterwards, the ceramic circuit board and the FPC are positioned at a predetermined place and subjected to thermo-compression bonding. Otherwise, the FPC is solder-plated before carrying out the above-described thermo-compression bonding.
Moreover, recently, adopting an anisotropic conductive film, an anisotropic conductive paste or an adhesive resin for the connection has been examined.
The anisotropic conductive film and the anisotropic conductive paste refer to a film and a paste providing an electrical connection that is conductive in a single direction and insulated in a direction perpendicular to the single direction. The electrical connection can be obtained by dispersing conductive particles in the resin film and the resin paste, and subjecting the resin to the thermo-compression bonding. Accordingly, when this anisotropic conductive film or the anisotropic conductive paste is used for configuring an adhesive layer, the circuit connector on the FPC and that on the ceramic circuit board are connected electrically to each other through the conductive particles in the anisotropic conductive film or the anisotropic conductive paste.
On the other hand, when the adhesive layer is configured using an adhesive resin having no conductive particles therein, the circuit conductor on the FPC and that on the ceramic circuit board are in direct contact with each other, and the circumference of the contact portion is adhered by the adhesive resin.
However, the above-described conventional method has the following problems.
First, connection between a ceramic circuit board and an FPC by using connectors is difficult, since the sizes and heights of large connector parts are quite unfavorable for the recent trend of miniaturization of modules, and pitches of circuit conductors to be connected become narrower. Moreover, the larger the number of connection pins, the more increased a price of the connector part. As described above, the conventional method is not suitable for manufacturing a high-density and miniaturized module in view of the cost performance and the performance capability.
Second, when solder is used for connecting the ceramic circuit board to the FPC, it is required that a connecting conductor on the ceramic circuit board or a connecting conductor on the FPC is soldered previously. At this time, a method of printing a cream solder on the connecting conductor on the ceramic circuit board is quite difficult to carry out, in the case where a pattern pitch becomes narrower than 500 μm. In a method of solder-plating the connecting conductor on the FPC, it is quite difficult to secure the soldering amount and accuracy at adequate degrees for the connection when the pattern pitch becomes narrower than 500 μm, which results in increased cost of FPC.
Even when soldering is conducted according to the method, a short can occur with adjacent patterns depending on connection conditions such as a temperature and a pressure. Thus, a stable connection condition is hard to find, and also the connection reliability varies in volume production.
Third, when any of the anisotropic conductive film, the anisotropic conductive paste or the adhesive resin is used for the connection between the ceramic circuit board and the FPC, at least about 150° C. of heat and about 30 kg/cm2 of a pressure are required. Accordingly, in the case of a connecting conductor with a narrow pitch, the pattern pitch of the FPC varies due to the heat and the pressure when the thermo-compression bonding is conducted in the case of a significant difference in thermal coefficients of expansion between the ceramic circuit board and the FPC. This results in defects such as a short with the adjacent connecting pattern on the ceramic circuit board occurring, which results in a difficulty of position adjustment between the ceramic circuit board and the FPC.